当数组大小存储在变量中时，查询 MongoDB 中数组的最后一个元素

Question

我在 MongoDB 中有一个数据集，这是我的一行数据的示例：

{ "conversionDate": "2016-08-01",
  "timeLagInDaysHistogram": 0,
  "pathLengthInInteractionsHistogram": 4,
  "campaignPath": [ 
      {"campaignName": "name1", "source": "sr1", "medium": "md1", "click": "0"},
      {"campaignName": "name2", "source": "sr1", "medium": "md1", "click": "0"},
      {"campaignName": "name1", "source": "sr2", "medium": "md2", "click": "1"},
      {"campaignName": "name3", "source": "sr1", "medium": "md3", "click": "1"} 
  ],
  "totalTransactions": 1,
  "totalValue": 37.0,
  "avgCartValue": 37.0
}

（campaignPath的长度不是固定的，所以每一行可以有不同数量的元素。

我想在campaignPath的最后一个元素中找到匹配“source = sr1”的元素。

我知道我不能用类似的东西进行查询

db.paths.find(
    {
        'campaignPath.-1.source': "sr1"
    }
)

但是，由于我存储的“pathLengthInInteractionsHistogram”等于campaignPath 长度的长度，我不能这样做：

db.paths.find(
    {
        'campaignPath.$pathLengthInInteractionsHistogram.source': "sr1"
    }
)

Answer 1

从 MongoDB 3.2 开始，您可以使用 aggregate 执行此操作，它提供了 $arrayElemAt 运算符，该运算符接受 -1 索引来访问最后一个元素。

db.paths.aggregate([
  // Project the original doc along with the last campaignPath element
  {$project: {
    doc: '$$ROOT',
    lastCampaign: {$arrayElemAt: ['$campaignPath', -1]}
  }},
  // Filter on the last campaign's source
  {$match: {'lastCampaign.source': 'sr1'}},
  // Remove the added lastCampaign field
  {$project: {doc: 1}}
])

在早期版本中，您无法使用$where。这会起作用，但性能很差：

db.paths.find({
    $where: 'this.campaignPath[this.pathLengthInInteractionsHistogram-1].source === "sr1"'
})

你也可以不用pathLengthInInteractionsHistogram：

db.paths.find({$where: 'this.campaignPath[this.campaignPath.length-1].source === "sr1"'})